Heretofore, a great deal of attention and effort has been directed to the development of polymeric containers for the storage of food stuffs and the like. Polyesters are currently being used for the packaging of food and beverages. Applications include polyester-coated paper board for ovenable trays, soft drink carbonated beverage bottles, mouthwash containers, thermoformed blister packs for the packaging of cold cuts, and films in various food wrap applications.
While there are plastics available which may be successfully utilized in hot-fill applications, these materials have one or more of several disadvantages, such as lack of acceptable strength, or the requirement of special processing techniques which considerably raise the manufacturing costs of the container, or the materials are too expensive to be cost-competitive.
As is well known, the major drawbacks in using polyester in hot-fill food applications are basically two-fold. First, if biaxially oriented, the polyester containers undergo considerable shrinkage when heated to temperatures at or near their glass transition temperature. Secondly, unoriented polyester containers will undergo a change in crystallinity at elevated temperatures such as those encountered in hot-fill food applications resulting in their becoming opaque and brittle.
Oriented polyester polymers, when heated above their glass transition temperature, tend to shrink back to an unoriented state. This shrinkage is caused by frozen-in stresses as a result of molecular chain extension during stretching. Therefore, in shrinking, the polyester exerts a retractive force. By heat setting a stretched polyester polymer under a constraint, oriented crystallites are formed. Total crystallinity increases without significantly affecting brittleness or clarity. This crystallinity persists up to near the heat setting or formation temperature. Shrinkage will therefore be curtailed or otherwise largely retarded during in-service use at elevated temperatures, such as in hot-fill applications.
This invention relates to a process for making polyester articles which includes heating an amorphous sheet of unoriented PET, drawing the PET sheet with a male plug, which causes biaxial orientation, transferring the drawn sheet from the male plug to a female mold which heat sets and molds the PET, then allowing the PET to shrink back onto the male plug which thereby cools the PET to below its glass transition temperature while being constrained by the male plug. A clear and non-brittle article is thus obtained.
By way of prior art are several United States and foreign patents, and several articles. U.S. Pat. No. 3,499,188 deals with an apparatus for forming articles of cold-strengthenable materials. There is no teaching in this patent concerning heat annealing polyethylene terephthalate.
U.S. Pat. No. 3,739,052 teaches a process for forming a thermoplastic container in which the cycle time has been reduced over the prior art. No mention is made of heat annealing the thermoplastic resin, thus this patent is not pertinent.
Belgian Pat. No. 872,272 deals with improvements and vessels for carbonated beverages. Again, no mention is made of heat annealing for hot-fill applications.
Dutch Pat. No. 8004-049 deals with deep drawn, thin-walled plastic containers formed by process of drawing a preheated sheet with a punch and shaping the blank in a die using air pressure. The invention utilizes a cold female mold cavity, and thus teaches away from the process in the instant invention.
British Pat. Nos. 1,374,969 and 1,374,970 concern a sealed, thin-walled container and process for making same. No mention is made of the use of polyesters or of forming biaxially oriented heat-annealed containers.
British Pat. No. 1,367,338 deals with a method for making cupped articles from a polyolefin plastic sheet. Again, no mention is made for biaxial orientation or heat annealing of polyester. Thus, this patent also is not pertinent.
British Pat. No. 1,508,574 concerns improvements relating to containers thermoformed in plastic materials. More specifically, the patent teaches a process for forming containers having nestable qualities and which resists storage creep. No mention is made of hot-fill capability, or heat setting.
Several articles by Shell Research deal with solid phase-pressure forming and solid phase-stretch forming for polypropylene. In particular, the article entitled "Polyproylene Solid Phase Pressure Forming a New Process for the Packaging Industry" by M. Ball, S. J. Dagniaux and K. G. Moxom, published in the SOCIETY OF PLASTICS ENGINEERS, ANTEC, 1974, deals with the thermoforming of polypropylene. "Solid Phase Pressure Forming Process for Thin-Walled Polypropylene Containers," J. M. Beijen is also concerned with strictly forming polypropylene containers. This article was published in PLASTICS AND RUBBER PROCESSING, June 1979. Neither of these articles deal with the biaxial orientation and heat annealing of polyester. An Industrial Information brochure published by Shell entitled "Polypropylene and Solid Phase Forming," explains in detail both the solid phase pressure forming and solid phase stretch forming. This article, like the two mentioned above, does not deal with the use of polyesters.
An article entitled "The Use of Polypropylene for a Thin-Walled Container Manufacturer," by S. J. Dagniaux and K. G. Moxom deals specifically with solid phase pressure forming and, as above, does not deal with polyesters.